This invention relates generally to devices which support and hold equipment and more particularly to a foot for such devices suitable for holding geometric equipment and accessories.
Although the art of surveying is old, recent improvements in equipment, such as automatic electronic total surveying stations, have increased the accuracy of the surveying instruments far beyond what was possible previously. Where accepted accuracy for equipment in the not too distant past was 20 seconds of a degree, accuracy is now commonly required to be 1 second of a degree. At these levels of precision, the instruments and many of their accessories must be supported in a manner which is consistent and stable. Accordingly, there is a need for closer examination of support structure used for surveying equipment so that improvements in instrumentation will not be lessened by inadequate supports. Further, the cost of surveying equipment makes it highly undesirable to damage it by failure of the supports because of instability.
Pole supports used in the surveying field typically take the form of bipods or tripods, but conceivably any number of legs could be provided. Conventionally, it has been necessary to provide completely separate inventories of bipods and tripods to meet the needs of different customers and for different applications. Legs of these supports are usually freely pivot able about a head of the support to swing toward each other for ease of carrying the support when not in use. A support having three or more legs can support equipment or other items above and out of contact with the ground. Typically, bipods are used to support surveying equipment such as a surveying pole, which also contacts the ground, but tripods may also be used to support surveying poles. The stability of such supports can be compromised in situations where there are forces (e.g., wind and loads from the surveying equipment) acting on the support and tending to tip the support over. In that case, one of the legs is subjected to an upward force component tending to raise the leg up as the support pivots about a point(s) of contact of the other leg(s) with the ground.
Presently, there is little or no resistance provided by the legs to such upward forces other than the weight of the leg. Even if there are stakes associated with the legs that penetrate the ground, the upward force tends to be directed right along the long axis of the stakes so that they are lifted out of the ground. The problem is compounded by the fact that the legs conventionally are mounted for pivoting freely about a horizontal axis. Even if the support does not tip over, if one leg loses contact with the ground or the frictional resistance to pivoting movement of the leg becomes sufficiently small, the leg will swing in toward the center of the support. The support is unlikely to regain stability when this happens. Should the support move back toward its original position, the leg having pivoted inwardly is no longer in position to engage the ground in a stable position. The center of gravity of the combined support and surveying equipment may lie outward of the point where the leg re-contacts the ground so that the entire unit topples over in the opposite direction from the initial tipping movement. Alternatively as the leg pivots inwardly, the weight of the leg exerts a smaller torque resisting the tipping motion so that the motion may continue causing the unit to collapse. If instability of the support causes it to move, even where it does not fall over, accuracy is compromised.
The legs of the support are most often telescoping so that their length may be adjusted as needed so that, for instance, the support can be set up on uneven terrain or the ends of the legs contact the ground at different leg angles. In the situation where the support is used with a surveying pole, the legs are manipulated so that the pole is precisely located over a selected point. When the telescoping legs are extended, it is necessary that they be locked at a specific length. Locking systems work most commonly by greatly increasing the frictional engagement of one leg section with the other. These systems suffer from certain drawbacks. Some locking systems may have a component of motion which is parallel to the lengthwise extent of the leg when engaged. This movement can cause the length of the leg to change during engagement of the locking system and result in loss of precision in the position of a surveying pole held by the support. The leg sections may be difficult to move relative to each other because of residual frictional engagement of the leg sections caused by the locking system, even when the locking system is released. In that situation movement may be jerky, making it difficult to extend or retract the leg to a precise length. Moreover, it is conventionally necessary to manually hold the locking device to keep the leg sections unlocked. This makes manipulation of the legs difficult, particularly where there are more than two legs. Conventionally, the leg sections are assembled at the factory and cannot be readily taken apart. Accordingly, cleaning and repair in the field is difficult.
Although the present invention has particular application for use in supporting surveying and geographic positioning equipment, it is not limited to such applications. As used herein “geometric” is intended to encompass surveying and geographic positioning. The invention is envisioned as being useful to support equipment and other items having no relation to surveying or geographic positioning.